Integrated circuit chips may be packaged in a variety of ways depending upon the performance and reliability requirements of the system in which they are used. High end integration schemes, sometimes referred to as multichip modules (MCM) or single chip modules (SCM), normally include at least one integrated circuit chip which is mounted to an insulating substrate. The insulating substrate, which may be ceramic, for example, has one or more wiring layers and thus provides a medium for electrical connections between chips (on an MCM) and/or between modules (for MCM or SCM). The wiring layers of the substrate are terminated at each of the top and bottom surfaces of the substrate in an array of I/O pads for interfacing to the chip and to a circuit board or other higher level module. The I/O pads may be a part of a controlled collapse chip contact (C4), ball grid array (BGA) or other connection scheme.
Substrates are typically tested prior to chip attachment in order to locate wiring errors or manufacturing defects. Shown in FIG. 1 is an exemplary substrate tester 10. The substrate tester 10 includes a supporting base 12, upon which is mounted positioning means 14, upon which is mounted an I/O contact assembly 16. Disposed above the I/O contact assembly 16 is probe assembly 18 and positioning means 20. The substrate to be tested is received by the I/O contact assembly 16. The positioning means 14 can move the x-y location (e.g. horizontal position) of the I/O contact assembly 16 for aligning the I/O contact assembly 16 with the probe assembly 18. The positioning means 20 can move the probe assembly 18 in the z-direction (e.g. vertically) to raise and lower the probe assembly 18 with respect to the I/O contact assembly 16. Controllers 22 provide signals to control the movement of positioning means 14 and 20, apply test signals to the substrate through the I/O contact assembly 16 and/or probe assembly 18 and measure the results.
With reference to FIG. 2, there is shown in further detail a portion of an exemplary conventional probe 30 for testing a substrate 26 that has an array of I/O pads 28. The probe block 30a forms a part of the probe assembly 18 shown in FIG. 1 and has an array of probe pins 24 arranged to individually contact the I/O pads 28. The I/O contact assembly 16 contacts the I/O pads (not visible in FIG. 2) on the underside of the substrate 26. In order to test for shorts/opens in the substrate 26 predetermined voltage levels are selectively applied to the I/O pads on the underside of the substrate 26 via the I/O contact assembly 16; the output voltages at the I/O pads 28 are then measured by the probe 30.
The probe assembly 18 is an effective, but expensive means of testing substrates 26. Spacings between I/O pads 28 may be as small as 75 micrometers for state of the art substrates, and will likely be even smaller in the future. Thus, the spacing between the probe pins must be of a similarly small magnitude. Additionally, the accuracy to which the probe pins must be located within the probe 30 is extremely high. Such accuracies are quite difficult to achieve for machined or molded articles, and thus make the fabrication of the probe 30 and probe assembly 18 very expensive. Multiplying the expense is the need for a customized probe for each type of substrate tested (e.g. I/O pin array is very unlikely to be same for any two substrate designs). Additionally, aligning the probe to each substrate to be tested requires precise aligning means, such as an optical alignment system, which can add expense in terms of equipment cost an/or reduced throughput. If integrated into the substrate tester 10 such an alignment system significantly limits throughput. An alternative approach is to use a separate alignment system called a mapper, which speeds up processing time, but requires a greater equipment investment. Furthermore, changing the customized probes each time a different substrate is encountered also significantly limits tester throughput.
Shorting pads have been proposed as an alternative to the probe of FIG. 2, for the more limited purpose of testing for undesired opens in the substrate. A shorting pad can be formed from a conductive material which is placed across a plurality of the I/O pads in order to short them together during the test. Applicants have observed a variety of problems have occurred with the use of shorting pads. Breakage of substrates has been a problem with certain type of shorting pads, such as those relying on a piece of conductive cloth stretched across a supporting frame or block to make the connection between I/O pads, because the pressure required to provide acceptable electrical continuity has been excessive. Another alternative is to spread conductive paste on a substantially flat probe tip. However, the paste does not stay on the probe tip, thus requiring cleaning of each substrate tested and frequent reapplication of the paste to the probe tip. While conductive elastomer shorting pads have not resulted in breakage and are apparently cleaner than conductive paste, they have their own set of problems. The applicants have discovered that conductive elastomer shorting pads can leave behind a residue on the substrate which is not easily removed. The residue left behind can include metals, such as silver, which can cause reliability problems. Metals, particularly silver, can migrate over time, under certain voltage, temperature and humidity conditions, thereby forming dendritic growths which can bridge across conductors (e.g. such as I/O pads) which would otherwise be electrically isolated, thus shorting together the conductors. The residue can also include oil, such as silicone oil, which makes the I/O pads non-wettable, thus rendering the substrate unusable. The residue is extremely difficult to remove.
What is needed is a probe which overcomes the problems discussed hereinabove.